1. Critical Length of Grade
Additional Material Provided for Your Benefit
Spring 2010

2. Critical Length of Grade
Maximum grade in itself is not a complete design control
“Critical length” is used to indicate the maximum length of a designated upgrade on which a loaded truck can operate without an unreasonable reduction in speed
Should consider using less steep grades or added lanes if the “critical length” is too long

3. Critical Length of Grade
The design values are determined by the following factors:
Size and power of vehicle: use weight/power ratio equal to 200lb/hp
Speed at entrance to the critical length:: should use the average running speeds of trucks (need to adjust for potential upgrades and downgrades)
Minimum speed on the grade below in which interfere to following vehicles is considered unreasonable: this depends on whether or not the facility is a two-lane or multilane highway; the time period other vehicles are unable to pass (note >= minimum 25 mph)

4. Crash Involvement Rate

5. Critical Length of Grade
A common basis for determining the critical length of a curve is based on the reduction in speed of trucks below the average running speed
In the past, 15 mph below the average would be acceptable. Now, should use 10 mph as the a rule of thumb (see previous graph)
Exhibit 3-59 can be used to estimate the critical length of curve

6. Critical Length of Grade
3-59

7. Critical Length of Grade
Speed 60 mph, 4%, reduction of 10 mph
1,200 ft
3-59

8. Critical Length of Grade
Example:
What is the critical length of a 5% upgrade approached by a 2% upgrade with a length equal to 1,650 ft?

9. Critical Length of Grade
6 mph
1,650 ft
3-59

10. Critical Length of Grade
325 ft
4 mph: 10 mph – 6 mph
3-59

11. Critical Length of Grade
Recreational Vehicle

12. Critical Length of Grade
Other design elements:
Must design for trucks that increase speed if the entrance is downgrade (10 to 15 mph)
When design controls are exceeded, need to consider adding an additional lane (next overhead)